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e-PS, 2013, 10, 77-82
ISSN: 1581-9280 web edition
ISSN: 1854-3928 print edition
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e-PRESERVATIONScience
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10
is published by Morana RTD d.o.o.
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THE INFLUENCE OF AIR EXCHANGE
ON THE STABILITY OF THE INDOOR CLIMATE
IN SKOKLOSTER CASTLE
Copyright M O R A N A RTD d.o.o.
Andrea Luciani1, Magnus Wessberg2, Tor Broström2
SCIENTIFIC PAPER
Abstract
e-Preservation Science (e-PS)
1. Politecnico di Milano, Department of
Architecture and Urban Studies (DAStU), Italy
2. Gotland University, Centre for Energy
Efficiency in Historic Buildings, Sweden
corresponding author:
andrea.luciani@mail.polimi.it
Skokloster Castle is a historic masonry building without any active climate control and hosting an important and heterogeneous collection of
artefacts. Despite being cited as a good passive preservation environment, conservators are observing decay in the collections related to the
indoor climate that may call for a re-evaluation of the climate control
strategy.
Air exchange is generally considered one of the driving forces influencing the indoor climate in unheated historic buildings. This study was
developed to better understand and evaluate its influence on the indoor
climate stability of the castle. The present study has outlined an experimental procedure for the assessment of the influence of air exchange
that can be used in historic buildings in general.
Air exchange rate was measured in seven rooms using tracer gas passive
sampling. The results were related to an analysis of the variability of
indoor temperature (T), relative humidity (RH) and mixing ratio (MR). A
connection with short-term RH fluctuations, considered the most dangerous for hygroscopic materials, was identified. Problems connected
with mould growth and high RH levels were also considered and discussed.
1
Introduction
Skokloster castle, located north of Stockholm, is a heavy stone and brick
building, completed in 1676 hosting an important and heterogeneous collection of artworks and objects (Fig. 1). The building envelope is characterized by masonry walls and lead glass windows with limited air tightness. An
inner corridor, which is in direct connection with outdoor air through the
staircases, connects the rooms. The building is open to visitors from May to
September. There is no active climate control system, with the exception of
a few rooms on the bottom floor used for offices. The factors influencing the
indoor climate of the castle are the heat and moisture transfer through the
envelope, the hygrothermal inertia of the structure and of the interiors and
solar radiation and wind.
Figure 1: View of the castle.
received: 13/03/2013
accepted: 10/06/2013
key words:
Air exchange, indoor climate stability, RH fluctuations, historic buildings
Despite being cited as a good passive preservation environment, conservators are now observing decay to the collections related to the indoor climate that may call for a re-evaluation of the climate control strategy. The main
problem is related to reoccurring mould growth in some rooms. A recent
condition survey of easel paintings has also showed considerable mechanical damage that has been associated with levels and variations of RH.
Gotland University has been monitoring the indoor climate of the castle
since 2009 in order to get a better understanding of the present indoor climate and what needs to be done about it.
A general survey of the indoor environmental conditions of the castle was
made by Broström and Leijonhufvud.1 The results showed that the yearly T
range in the rooms was between -7.4 °C and 29 °C inside, between -15 °C
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and 35 °C outside. Annual RH average of all rooms was
70%, 9% lower than outside. RH variations detected
inside the castle were reduced when compared to the
outdoor climate. For all rooms, the average seasonal
variation inside was 48% as compared to 85% outside;
the minimum values of RH detected in the rooms were
in the range of 33 to 56%, while the maximum values
were in the range of 87 to 97% (Figs. 2 and 3). There
were considerable differences among the rooms in
terms of short term variations affecting hygroscopic
materials. Air exchange was identified as one of the
main factors determining this situation. A reduction of
the air exchange rate was suggested as a strategy to
limit RH fluctuations. On the other hand, one of the
main problems affecting the collections was related to
high indoor RH levels, leading to mould growth in
some rooms, as described by Bjurman and
Leijonhufvud.2
2
Materials and methods
2.1
Indoor climate measurements
The study is based on data collected in Skokloster
Castle by previous research projects and analyses performed by Gotland University.1 The monitoring campaign was carried out from the 15th of June 2008 to
the 9th of September 2010 in 25 different rooms outspread on different floors and in different directions. 45
data loggers of the type TinyTag plus2 were used
during the campaign. Temperature (T) and relative
humidity (RH) data for the whole year were obtained
by the monitoring campaign. The corresponding
mixing ratio (MR) was then calculated.
2.2
Measurement of the air exchange rate
The air exchange rate was expressed as Air Changes
per Hour (ACH). Measurements were made following
the Nordtest Method standard NT VVS 188:1997:3
homogeneous emission of tracer gas was used to
measure the local mean age of air, utilising passive
tracer gas sources and diffusive samplers. The measurement is based on the fact that the local steady concentration of a contaminant distributed with equal
source strength in all parts of the space will attain a
value which is proportional both to the local mean age
of the air and the contaminant source strength per
volume. The inverse of the local mean age of the air in
a zone is defined as the ‘room-specific ventilation flow
rate’ or ‘local air change rate’. In naturally ventilated
environments, this parameter, expressed as ACH, can
be considered the equivalent of the specific ventilation
flow rate, usually used for the ventilated system as a
whole to express the flow rate of outside air entering
into a ventilated room, divided by the volume of the
ventilated room.
The present paper will therefore analyse the general
effect of air exchange on the hygrothermal parameters
inside the building focusing on its influence on the stability of the indoor climate and trying to understand
whether an improvement of the air tightness of the
building envelope could be beneficial without increasing the mould risk.
Measurements were made according to the following
schedule:
Autumn: from 2009-10-10 to 2009-11-23 – 44 days.
Winter: from 2010-01-21 to 2010-03-04 – 42 days.
Spring: from 2010-04-21 to 2010-05-27 – 36 days.
Summer: from 2010-07-14 to 2010-08-23 – 40 days.
Figure 2: Yearly trends of RH (%) and T(°C) in Room 2V (dark grey) compared to outdoors (light grey).
Figure 3: RH in twelve selected rooms: seasonal range (light grey),
range of a 30-day moving average (dark grey). The line in the middle of
each bar is the seasonal average.
Figure 4: Plan of the first floor of Skokloster Castle. The analysed rooms
are highlighted.
Indoor climate in Skokloster Castle, e-PS, 2013, 10, 77-82
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© by M O R A N A RTD d.o.o.
On the basis of the results obtained, seven rooms on
the second level (first floor) of the castle were selected
and investigated during this study (Fig. 4).
after (Fig. 5). A RH fluctuation from the seasonal average is considered outside the safe range when its magnitude is more than 1.5 the standard deviation calculated over 13 months.
2.3
In this study the method was adapted to analyze the
influence of air exchange on indoor climate fluctuations. The difference between each surveyed value
and the corresponding 30-day moving average value
was calculated, while standard deviation of the differences was calculated on the same period of days on
which air exchange measurements were performed.
Procedure to evaluate the influence of air
exchange on fluctuations
In order to evaluate the effect of the air exchange rate,
expressed as the average ACH in the surveyed periods,
it was necessary to find a comparable indicator for
temperature, relative humidity and the mixing ratio in
the same seasonal periods. The standard deviation (σ),
was chosen as a proper indicator of the general variability of the parameters in a given period of time. In the
seven selected rooms, standard deviation of T, RH and
MR was thus calculated for the same period for which
the corresponding average ACH was determined.
In order to show to what extent outdoor fluctuations
were transmitted to and/or buffered in the indoor
environment a specific parameter was created, the
Climate Fluctuations Transmittance (CFT). It expresses
the ratio between the standard deviation of the indoor
fluctuations and the standard deviation of the outdoor
fluctuations:
Graphs comparing ACH and σ of T, RH and MR in the
same seasonal periods were plotted to see if it was
possible to find a correlation between those values,
according to the following hypothesis:
CFTn1 = σ n 1 (Xin - MAn2 · Xin) / σ n 1 (Xout-MAn2 · Xout)
where σ is the standard deviation, MA is the moving
average, Xin are the indoor values (T or RH or MR), Xout
are the outdoor values (T or RH or MR), n1 is the number of days for which σ is calculated (36-44 days in
ACH measurements) and n2 is the number of days for
which MA is calculated (30 days following the EN15757
standard).
σ n ( T ) = ƒ T ( A CH n )
σ n ( RH, ) = ƒ R H ( A C H n )
σ n ( M R) = ƒ M R ( A C H n )
where n is the number of days on which both σ and
average ACH are calculated. The first results have suggested to focus this type of analysis on mixing ratio,
since it is not dependent on temperature and, compared to temperature and relative humidity, it is the parameter less influenced by solar radiation, heat gains or
thermal properties of the walls. It is thus considered a
good indicator to detect air movements in indoor
environments.4
As in the procedure to evaluate the general influence
of air exchange rate on hygrothermal parameters, CFT
was plotted with ACH in order to find a correlation:
C FT n (T)= ƒ T (A C H n )
C FT n (R H )= ƒ R H (A C H n )
C FT n (MR )= ƒ M R (A C H n )
The next step was to focus the analysis on relative
humidity, given its importance for the conservation of
hygroscopic materials. In this case we have to find a
valid method to distinguish seasonal variations from
short-term fluctuations, which were supposed to be
more related to air exchange and, at the same time,
potentially risky for hygroscopic materials. The EN
15757:2010 standard5 suggests a procedure to identify
RH short-term variations by the calculation of the difference between the current value and a central 30day moving average value, considering the values surveyed in the fifteen days before and in the fifteen days
where n is the number of days on which both average
ACH and σ in the CFT are calculated. Given the importance of RH fluctuations for the conservation of
hygroscopic materials, results regarding CFT will be
presented for relative humidity only, but the method
(as well as the EN15757 procedure) can also be applied
to the other parameters, i.e. temperature and mixing
ratio.
Linear regression was carried out and the Pearson’s
correlation coefficient (r) was calculated: the closer
the value is to 1, the stronger the correlation.
3
Results and discussion
3.1
General influence of air exchange on
hygrothermal parameters
The air exchange in the analyzed rooms generally ranges from 0.2 to 0.9 ACH, depending on the location of
the room and the time of the year (Fig. 6). Higher values were detected in the corridor (1-2 ACH) and sometimes in the towers and corner rooms that are more
exposed to outdoors.
Nevertheless it must be underlined that values detected do not represent exclusively air infiltration from
outdoors but also air movements between the rooms.
This bias is introduced by the method chosen for the
tracer gas tests: with the single-gas sampling technique, not only the air exchange between outdoor cli-
Figure 5: Yearly graph of RH in room 2V (dark grey). The RH monthly
moving average (thick orange line) and the safe RH range according to
the EN15757 standard are also included.
Indoor climate in Skokloster Castle, e-PS, 2013, 10, 77-82
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Figure 6: Air exchange measurements expressed as ACH in first floor
rooms in different seasons. The towers (blue) and the corridor (cyan)
have the highest values.
Figure 8: CFT of RH as a function of ACH. Different seasons are highlighted. Linear regression is used to show the correlation coefficient
considering all the values for the whole year (r = 0.73).
Figure 7: Standard deviation of indoor MR as a function of ACH in different seasons. Different seasonal patterns are highlighted. Air exchange
seems to have an higher influence on MR variations in spring and summer (r = 0.91 and 0.79, respectively) than in autumn or winter (r = 0.47
and 0.28, respectively).
Figure 9: CFT of RH as a function of ACH. Linear regression is used to
show the correlation coefficient considering the values of the four most
similar rooms for the whole year (r = 0.86).
mate and indoor climate is measured, but also the air
exchange between the various rooms of the buildings
(e.g. the round corridor). A double-sampling technique
could be helpful in solving this problem by distinguishing the indoor air circulation from air leakage or infiltration through the envelope.
The graph comparing ACH values with MR variations,
expressed as standard deviation, highlights different
seasonal patterns (Fig. 7). Autumn and winter values
have flatter trends while similar ACH values correspond to larger variations in spring and summer. The
strongest correlation was found in spring (r = 0.91). In
summer we have a good correlation as well (0.79)
while it seems poorer in autumn and winter (r = 0.47
and 0.28, respectively). On the one hand this should be
due to seasonal differences in the magnitude of MR.
On the other hand the results could also be influenced
by the fact that in spring and summer the castle is
open to visitors and windows are more likely to be
opened. As a consequence, the amount of outdoor air
on the total exchanged air should be higher and this
could result in a stronger influence of air exchange on
MR variations in summer and spring, although the
Figure 10: The duration graph is a cumulative presentation of RH levels
in rooms 2H, 2Q, 2R and 2V. A flatter curve indicates a more stable climate.
Indoor climate in Skokloster Castle, e-PS, 2013, 10, 77-82
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© by M O R A N A RTD d.o.o.
amount of air changes per hours is generally similar to
the autumn and winter one.
3.2
Influence of air exchange on RH
fluctuations
In Fig. 8 the climate fluctuations transmittance of RH
was plotted as a function of ACH. In this case seasonal
patterns are not so evident and a strong correlation (r
> 0.7) can be found between the CFT of RH and the air
exchange. This indicates that in Skokloster Castle,
regardless of seasonal trends and the position and
orientation of the rooms, RH short-term fluctuations
are directly connected to air exchange. A higher air
exchange rate is therefore expected to cause a higher
level of fluctuations and, as a consequence, a potential
higher risk for the hygroscopic materials in the collections. If we consider only the rooms with similar characteristics, such as shape, dimensions, ratio between
volume and surface exposed outdoors (Fig. 9) there is
an even stronger correlation. For all the seven analysed
rooms during the four seasons, the Pearson correlation coefficient was 0.73. The same coefficient calculated on four more similar rooms was 0.86. This would
suggest that, despite other factors have an influence
on RH fluctuations (e.g. temperature variations or moisture
transfers),
the
Climate
Fluctuations
Transmittance can give good results if used as an indicator in order to evaluate the influence of air exchange on RH fluctuations in different indoor environments.
Figure 11: Mould growth risk evaluation in rooms 2H, 2R, 2V. Values
above the red line may result in mould growth.
Figure 12: Difference between outdoor dew point and indoor temperature in room 2V. Values close to or below 0 °C have to be considered
risky if outdoor air infiltrates into the room.
The results obtained are confirmed by duration curves,
as shown in Fig. 10. Yearly indoor values of RH fluctuations in four rooms with different ACH, different
shape and different orientation, were plotted in a
cumulative graph. The two rooms with a lower air
exchange rate (i.e. 2H and 2R) show flatter curves and
lower peaks if compared with the two rooms with
higher air exchange rate (i.e. 2V and 2Q), indicating a
higher stability of RH levels.
rooms, regardless of the different RH stability and ACH
shown in Fig.10.
Sedlbauer isopleths6 were used to describe the mould
risk in the rooms (Fig. 11). Values above the red line can
result in mould growth if given time long enough.
Room 2H has the lowest air exchange rate and has no
mould growth risk, while room 2V has the highest air
exchange rate and a significant mould growth risk.
Room 2R has a low air exchange rate and at the same
time quite a high risk. A correlation between air
exchange and mould growth risk is thus not implied
but it can be claimed that in analysed rooms a lower air
exchange rate does not necessarily result in a higher
mould growth risk.
With regard to the observed influence of strong wind
in RH drops or peaks, wind speed was measured and its
variations were compared with RH indoor trends in
some rooms in order to find some connections, but
results are still not significant and a deeper analysis is
probably needed to obtain a better comprehension of
this issue.
3.3
Expected influence of a reduction of air
exchange on mould growth and on RH
high levels
With high humidity levels outdoors and low temperatures indoors, the outdoor dew point can fall below
the indoor temperature, which would mean a potential
risk of condensation of outdoor air entering the rooms
(since the rooms are unheated surface temperatures
are generally in equilibrium with air temperatures). In
such a situation, a lower air exchange rate could reduce the risk. Fig. 12 shows the difference between the
indoor T in room 2V and the outdoor dew point.
It is common to associate low air exchange rates with
moisture problems. However, it is not necessarily so. In
environments like the analysed rooms of Skokloster
castle, where there are no internal moisture sources,
the difference between indoor and outdoor MR is
generally small throughout the year. Thus mould risk
would depend on differences in T and RH over the
short term. A previous study showed that a key factor
for mould risk seems to be the temperature levels in
the rooms.2 As a consequence, north-facing rooms
(2R, 2V) with less solar gain, have lower T and higher
RH. This results in a higher mould growth risk and
there have been mould problems indeed in both the
Anyway, the current strategy of passive climate control
is not sufficient to keep RH below dangerous levels in
rooms in the north part of the building and an active
intervention is probably required. A comparative experimentation of different climate control strategies
(dehumidification, conservation heating or adaptive
ventilation) is currently going on in some rooms to
Indoor climate in Skokloster Castle, e-PS, 2013, 10, 77-82
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3. NT VVS 118:1997, Ventilation: local mean age of air –
Homogeneous emission techniques, NORDTEST, Espoo, 1997.
establish the more effective and energy-efficient
method to reduce the mould growth risk.
4. D. Camuffo, Microclimate for Cultural Heritage, Elsevier,
Amsterdam, 1998, 48-53.
4
Conclusions
5. EN15757:2010, Conservation of Cultural Property - Specifications
for temperature and relative humidity to limit climate-induced
mechanical damage in organic hygroscopic materials, European
Committee for Standardisation, Brussels, 2010.
The study has assessed the effect of air exchange on
the hygrothermal parameters inside the building focusing on its influence on the stability of the indoor climate and trying to understand whether an improvement of the air tightness of the building envelope
could be beneficial without increasing the mould risk.
A specific method was developed and a new parameter, called Climate Fluctuations Transmittance, was
proposed.
6. K. Sedlbauer, Prediction of mould growth by hygrothermal calculation, J. Therm. Envelope Build. Sci., 2002, 25, 321-336.
Based on measurements from Skokloster castle, the
present investigation has shown that a reduction of the
air exchange rate can provide a more stable indoor climate. A correlation between air exchange and RH
short-term fluctuations was found, indicating that a
reduction of the air exchange rate could be an effective strategy to lower the risk for mechanical damage on
hygroscopic materials.
It is generally thought that an improvement of air
tightness can cause a higher mould growth risk.
However, this study shows that in a building like
Skokloster Castle, with no internal moisture sources,
this is not necessarily true. Nevertheless, passive control is probably not enough to lower dangerous RH
levels in rooms in the north part of the building and
active control is probably necessary.
The Climate Fluctuations Transmittance has proved to
be effective for the purposes of this study. Results
could be improved by a deliberately selected dataset,
based on the most suitable rooms and on the most significant periods to be analysed. Having the same 30day period of ACH measurements for each season
would make the data more coherent and fully corresponding with the calculated moving average
In conclusion, the method proposed in this study
through the use of the Climate Fluctuations
Transmittance parameter can be used as a general
procedure to assess the influence of air exchange in
unheated historic buildings. Nevertheless further
applications to other case studies are needed to validate the method.
5
Acknowledgements
The investigations in Skokloster have been carried out
with the support of the Swedish Energy Agency as part
of the national research programme on energy efficiency in historic buildings (Spara och bevara).
6
References
1. T. Broström, G. Leijonhufvud, The indoor climate in Skokloster
Castle, in: D. Del Curto, ed., Indoor environment and preservation.
Climate control in museums and historic buildings, Nardini Editore,
Firenze, 2010, 75-88.
2. J. Bjurman, G. Leijonhufvud, An Analysis of Microclimate
Differences Leading to Sporadic Mould Growth in Skokloster Castle,
an Unheated Historic Building, in: T. Broström, L. Nilsen, eds.,
Postprints from the Conference Energy Efficiency in Historic Buildings.
Visby, February 9–11, 2011 Gotland University Press, Visby, 2012, 236244.
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